Electrode for thin film capacitor devices

ABSTRACT

A method of forming a conductor on a substrate including steps of depositing tantalum on a glass layer of the substrate; oxidizing the tantalum; and depositing a noble metal on the oxidized tantalum to form the conductor. The method can be used to form a ferroelectric capacitor or other thin film ferroelectric device. The device can include a substrate comprising a glass layer; and an electrode connected to the glass layer. The electrode comprising can include a noble metal connected to the glass layer by an adhesion layer comprising Ta 2 O 5 .

GOVERNMENT RIGHTS

This invention was made with government support under contract No.F33615-98-2-1357 awarded by the United States Department of the AirForce. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to thin film capacitor devices orferroelectric devices and, more particularly, to a method of forming anelectrode on a glass layer of a substrate.

2. Brief Description of Prior Developments

Platinum (Pt) is commonly used as an electrode material for thin-filmferroelectric devices. This is due to its resistance to oxidation attemperatures above 600° C. in oxygen (O₂); an environment which isrequired to obtain good electrical characteristics from materials suchas (BaSr)TiO₃ (BST), (PbZr)TiO₃ (PZT), SrBi₂Ta2O₉ (SBT), SrBi₂Nb₂O₉(SBN), SrBi₂(TaNb)₂O₉ (SBTN) and others. Devices using these materialsare frequently made using oxidized silicon wafers as a substrate.

Adhesion of a platinum electrode to the oxide of the silicon wafer hasalways been a problem. The problem has been exacerbated due to theunique properties of platinum and challenges of thin film ferroelectricprocessing. Platinum has a tendency to plastic flow in response tostress at temperatures above 600° C. Ferroelectric thin film depositionis frequently performed using spin-on methods which result in a veryhigh tensile stress due to film shrinkage as it is sintered. Theferroelectric material can contain metals such as Bi. These metals canalloy with the platinum.

The ferroelectric processes often incorporate long anneals in O₂ atelevated temperature to improve performance. Even if the platinumadheres to the substrate, these processes frequently cause the formationof hillocks or bumps on the surface of the platinum electrode. Thesehillocks can cause localized high electric fields, high leakage, andearly breakdown. In more severe cases, hillocks can be large enough todirectly short out the ferroelectric device or combine with otherdefects in the film to short out the devices resulting in reduced yieldor unusable devices.

Historically a thin film of titanium (Ti) has been added between theplatinum and the oxide of the silicon wafer. This titanium layer isabout 10% of the thickness of the platinum electrode and greatlyimproves adhesion of the platinum to the oxide. An example of thiselectrode is given in U.S. Pat. No. 5,723,171. The use of a titaniumadhesion layer has two major problems. First, the process windows arevery narrow. Changes in anneal times or temperatures or changes in thethickness of the ferroelectric often require a re-optimization of thetitanium and platinum layer thicknesses. Second, titanium is very mobileand can migrate through the platinum electrode causing degradedperformance of the ferroelectric layer. This is a known problem withBST, SBT, SBTN and others where variations in film composition, due totitanium incorporation degrade film performance.

Other methods to improve adhesion have been employed with a wide varietyof metallic layers such as Cr, Ta, Vd, Nb, Sr, Ru, Os, Pd (see U.S. Pat.Nos. 6,103,400 and 6,054,311). While these other metals may work oversome range of conditions, these processes all suffer from either pooradhesion or from volume expansion of the adhesion layer due to oxidationduring high temperature oxygen anneals which results in an unstablefoundation for the platinum and limits their utility.

SUMMARY OF THE INVENTION

In accordance with one method of the present invention, a method offorming a conductor on a substrate is provided comprising steps ofdepositing tantalum on a glass layer of the substrate; oxidizing thetantalum; and depositing a noble metal on the oxidized tantalum to formthe conductor.

In accordance with another method of the present invention, a thin filmferroelectric device is provided comprising a substrate comprising aglass layer; and an electrode connected to the glass layer. Theelectrode comprises a noble metal connected to the glass layer by anadhesion layer comprising Ta₂O₅.

In accordance with one aspect of the present invention, a thin filmferroelectric device is provided comprising a substrate comprising aglass layer; and an electrode connected to the glass layer. Theelectrode comprises a noble metal connected to the glass layer by anadhesion layer comprising Ta₂O₅.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the present invention areexplained in the following description, taken in connection with theaccompanying drawings, wherein:

FIG. 1 is a schematic cross sectional diagram of a componentincorporating features of the present invention;

FIG. 2 is a schematic cross sectional diagram of the component shown inFIG. 1 before the electrode is formed;

FIG. 3 is a process flow chart of steps used to form the component shownin FIG. 1;

FIG. 4 is a diagram of some of the devices used to form the componentshown in FIG. 1; and

FIG. 5 is a schematic cross sectional diagram of a ferroelectriccapacitor incorporating features of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a diagram of a cross section of asubcomponent 10 incorporating features of the present invention.Although the present invention will be described with reference to theexemplary embodiment shown in the drawings, it should be understood thatthe present invention can be embodied in many alternate forms ofembodiments. In addition, any suitable size, shape or type of elementsor materials could be used.

The subcomponent 10 is generally intended to be used in fabricating alarger electrical component, such as a thin film ferroelectric device.For example, the component 10 could be used to fabricate a ferroelectricdevice, such as a ferroelectric capacitor 12 (see FIG. 5). Thesubcomponent 10 generally comprises a substrate 14 and an electrode 16.

The substrate 14, in the embodiment shown, is preferably a semiconductorwafer. However, in alternate embodiments, any suitable type of substratecould be provided. The substrate 14 generally comprises a base 18 and atop layer 20. The base 18 preferably comprises a 3-15 ohm-cm Phosphorousdoped silicon substrate. However, any suitable type of base could beprovided. In the embodiment shown, the top layer 20 is comprised ofglass. In a preferred embodiment, the glass layer 20 comprises silicondioxide (SiO₂). For example, the glass layer 20 could comprise 5000 Åthick wet silicon oxide. However, in alternate embodiments, thesubstrate 14 could be comprised of any suitable type of a top layer.

The electrode 16, in the embodiment shown, generally comprises anadhesion layer 22 and a electrical conductor layer 24. The adhesionlayer 22 is preferably comprised of tantalum pentoxide (Ta₂O₅). Theadhesion layer could be selected from a group consisting of tantalumoxide, titanium oxide, zirconium oxide, hafnium oxide and vanadiumoxide. The electrical conductor layer 24 is comprised of a noble metal,preferably platinum. The noble metal conductor could be selected from agroup consisting of platinum, palladium, gold and rhodium. Theelectrical conductor layer 24 is attached to the top player 20 of thesubstrate 14 by the adhesion layer 22.

Adhesion of platinum (Pt) to glasses, such as silicon oxide (SiO₂), hasbeen a longstanding problem. Traditionally, the adhesion issue has beendealt with by including a thin metallic adhesion layer between theplatinum and the glass. This layer is typically composed of Ti, Cr, Ta,Va, Nb, Pd, Ir, Ru and is typically about 10% of the thickness of theplatinum layer. Use of such adhesion layers results in very narrowprocess windows if the platinum electrode is to survive in oxidizingenvironments at temperatures above about 600° C. This is due to acombination of the tendency of platinum to plastic flow at thesetemperatures in response to stress and the poor performance of theplatinum as a barrier to oxygen, which allows oxidation of the adhesionlayer causing a volume expansion. This means that the typical metallicadhesion layer is an unstable surface for the platinum in thisenvironment.

The most commonly used adhesion layer, titanium (Ti), has the furtherproblem of migrating through the platinum electrodes. When used in anelectrode for thin ceramic films, this causes stoiciometry changesand/or contamination issues which can result in degraded performance.Examples of this degradation include a reduction in relativepermittivity for BST and reduced remnant polarization in SBTN or PZT.

The new electrode system of the present invention combines heateddeposition, to control stresses in the metal layers, and oxidation ofthe adhesion layer before depositing the conductive layer. Oxidation ofthe adhesion layer before depositing the conductive layer preventsvolume expansion from occurring during subsequent thermal processing.This results in a smooth, highly adherent surface which remains smoothduring subsequent processing. Further, these electrodes show littlesensitivity to variations in thickness of either the platinum or theadhesion layer, little sensitivity to the purity of the depositedmaterials, and little sensitivity to anneal processes. This allows useof thicker electrodes which, for many applications, are required forincreased power/current handling or reduced resistive loss. Theseelectrodes of the present invention remain smooth and planar even whenexposed to very high compressive stresses and through anneals as long as10 hours at 725° C. in oxygen. These electrodes also show higher remnantpolarization (2Pr) in SBTN and lower leakage and higher breakdown fieldsin BST.

Referring also to FIGS. 2 and 3, FIG. 2 shows the subcomponent 10 ofFIG. 1 before the electrical conductor layer 24 is formed thereon. FIG.3 illustrates some of the method steps used to form the subcomponent 10.As shown by block 26, tantalum is deposited on the top glass layer 20.After the tantalum is deposited on the top glass layer 20, the tantalumis then oxidized as illustrated by block 28. This forms the assembly asshown in FIG. 2. With the tantalum oxide 22 attached to the top glasslayer 20, a noble metal (in this embodiment Pt) is deposited on thetantalum oxide as illustrated by block 30.

Referring also to FIG. 4, in a preferred method, an evaporator 32 isused for heated deposition of the tantalum and the platinum on the waferwhich eventually forms the subcomponent 10. However, in alternateembodiments, the tantalum and the platinum could be deposited on thewafer by any suitable method, and perhaps by different methods. Theevaporator 32 is preferably an e-beam evaporator which can heat thewafer 15 in a vacuum. However, in alternate embodiments, any suitableevaporator or metal deposition method could be used. In order to oxidizethe tantalum, the system comprises a rapid thermal processor (RTP) 34.However, in alternate embodiments, any suitable device for oxidizing thetantalum could be used, including in-situ oxidations within the metaldeposition system by the introduction of oxygen, direct evaporation ofTA₂O₅ or reactive sputter of TA₂O₅ in an O₂ environment. The metal oxideadhesion layer may be deposited by direct sputtering of a metal oxidetarget; by vacuum deposition using MOCVD of an appropriate precursor; byspin-on deposition and firing of a MOD or sol-gel precursor; or by anymethod suitable for depositing metal oxide films. The rapid thermalprocessor can expose the wafer 15 to an elevated temperature for apredetermined time in an oxygen ambient atmosphere. This processcompletely oxidizes the metallic tantalum layer.

After the tantalum layer has been oxidized to form the tantalum oxidelayer 22, the wafers 15 are then placed back into the evaporator 32 andheated to a predetermined temperature in a vacuum where the noble metal(Pt in the embodiment shown) layer can be deposited. In a preferredembodiment, the predetermined temperature is about 310 degrees Celsius.However, in alternate embodiments, any suitable temperature could beused. In a preferred embodiment, the platinum layer 24 has a thicknessof about 2500 angstroms. However, in alternate embodiments, the platinumlayer could have any suitable thickness. For example, the electricalconductor layer 24 could be from about 1600 angstroms to about 3500angstroms.

Referring now also to FIG. 5, the subcomponent 10 is shown as part of aferroelectric capacitor 12. The capacitor 12 generally comprises thesubcomponent 10, a ferroelectric layer (FE) 42, a top electrode (TE) 44,an interlayer dielectric (ILD) 46, and an interconnect metal (M3) 48. Inthe embodiment shown, the electrode 16 forms the bottom electrode forthe capacitor 12. In a preferred embodiment, the electrode 16 is about2500 Å. However, in alternate embodiments, the electrode could have anysuitable thickness.

In the preferred embodiment shown, the ferroelectric layer 42 is about2000 Å, the top electrode 44 is about 1100 Å, the inter layer dielectric46 is about 3000 Å, and the interconnect metal 48 is about 1850 Å.However, in alternate embodiments, these components could have anysuitable thickness. The ferroelectric thin film layer 42 could comprisebarium titanate, strontium titanate, barium strontium titanate, leadzirconate titanate, strontium bismuth tantalate or strontium bismuthtantalate niobate. In the preferred embodiment shown, the top electrode44 is comprised of platinum. However, in alternate embodiments, the topelectrode 44 could comprise any suitable type of electrical conductormaterial. In the embodiment shown, the interlayer dielectric 46 iscomprised of low pressure chemical vapor deposition (LPCVD) SiO₂.However, an alternate embodiments, the interlayer dielectric could becomprised of any suitable type of material and formed by any suitablemethod. In the embodiment shown, the interconnect metal 48 is comprisedof platinum. However, in alternate embodiment, the interconnect metal 48could be comprised of any suitable type of electrical conductormaterial.

The present invention can provide improved performance of thin-filmdielectric and ferroelectric devices. Both leakage current and breakdownvoltage of thin insulating films are dependant on the morphology of theunderlying electrode. This invention allows the formation of a stable,smooth, and highly adherent electrode even with the relatively largethermal budgets required to produce high-quality ceramic thin films.Traditional films are marginal in adhesion and have a tendency to form arough hillocked surface when exposed to high temperature oxidizingenvironments and high stresses.

This new adhesive layer comprises a metal oxide, for example Ta₂O₅, suchas formed by evaporating tantalum metal and oxidizing it in a rapidthermal processor, as a stable adhesion layer before deposition of theplatinum (or other noble metal) electrode. When this is combined withheated deposition of the platinum to control stresses, the electroderemains planar and very smooth over a wide range of anneal and processconditions. This electrode also appears to be quite insensitive toprocess variations such as thickness of the tantalum layer, tantalumanneal conditions, thickness of the platinum layer, and age of theplatinum melt.

Ferroelectric devices fabricated on this new electrode show improvedyield, higher breakdown resistance, and improved electrical propertieswhen compared to previous conventional electrode structures. Use of thiselectrode structure has proven important to work on a BST varactor (suchas described in U.S. Pat. No. 6,101,102) which requires very thickelectrode layers to minimize resistive losses and high breakdown fieldsto achieve a wide tuning range and high current density.

In a preferred embodiment the substrate is a silicon wafer which hasbeen wet oxidized to produce a 5000A thick SiO₂ film. The wafer is thenplaced in an e-beam evaporator and heated to a temperature of 250° C. ina vacuum. A tantalum layer, approximately 200A thick, is then evaporatedonto the heated substrate. The system is vented and the wafer is allowedto cool. Neither the wafer temperature during evaporation nor thethickness of the tantalum layer are critical. The wafer is then removedfrom the system and the tantalum metal is oxidized by using a rapidthermal processor (RTP) to expose the wafer to a temperature of 725° C.for two minutes in an oxygen ambient atmosphere. Again, neither the timenor the temperature of this process is critical as long as the tantalumlayer is fully oxidized.

Good results were achieved for anneal times from 1 to 7 minutes. Theanneal temperature of 725° C. was chosen for convenience since 725° C.is the normal anneal temperature used for ferroelectric films. Othertemperatures were not explored, but certainly could be possible. Whileuse of an RTP to perform this oxidation was initially chosen forconvenience, it is likely that the use of a RTP to perform this annealmight be preferred. Long furnace tube anneals may result in a smootherTa₂O₅ surface with a large grain size which platinum may not adhere tovery well.

After the tantalum layer has been oxidized, the wafer is then placedback into the evaporator and heated to a temperature of 310° C. in avacuum where a platinum layer about 2500A thick is deposited. Thethickness of the platinum layer is not critical. In tests, there hasbeen success with platinum layer ranging from 1600A to 3500A and greaterwith this process. The temperature of the wafer during this depositionmight be important due to its effect on the tensile stress of the wafersafter cooling. Depositions at temperatures at or above 300° C. show animprovement in surface smoothness and adhesion compared to lowertemperature depositions. This result in essentially featureless surfacesat magnifications up to 1000×. In a test of the process, maximumdeposition temperature was limited by VITON™ seals in the vacuum system,so significantly higher temperature depositions have not been explored,but may be possible.

After venting and cooling, the wafer is removed from the vacuum system.At this stage the wafer is reasonably adherent and smooth. If necessary,improved adhesion can be obtained by a brief exposure to a highertemperature such as 30 seconds at 725° C. in an RTP, either as aseparate anneal on the electrode only, or combined with thecrystallization anneal on a subsequently deposited ferroelectric layer.The electrode surface will remain very smooth through the remainder ofthe ferroelectric processing including anneals totaling 10 hours at 725°C.

These improved electrodes have been tested with several ferroelectricmaterials including Bi containing ferroelectric films (SBT(N)) and BSTfilms having very high tensile stress. The devices made with theimproved electrodes have improved ferroelectric performance, leakagecurrents, and greater current handling ability. The use of metal oxidessuch as Ta₂O₅ as an adhesion layer for platinum electrodes used withceramic thin films is not known to exist in the prior art.

As an overview of a capacitor array process, the process can comprisesthe following steps:

-   -   Prepared starting wafer with 5000 Å wet oxidation    -   Deposit Bottom Electrode (BE)    -   Spin On Ferroelectric Precursor And Fire (FE)    -   Deposit Top Electrode (TE)    -   Etch Capacitor Stack    -   Test Point 1 To Verify Good Ferroelectric Properties    -   Deposit Interlayer Dielectric (ILD)    -   Open Contacts (CT)    -   Deposit Interconnect Metal (M3)    -   Etch Interconnect Metal    -   Test Point 2 To Screen For Yield    -   Deposit Overglass    -   Open Pads (GL)    -   Final Parametric Test

As an overview of the improved bottom electrode process of the presentinvention, the process can comprise the following steps:

-   -   deposit Ta adhesion layer        -   for example, 200A evaporated into wafer at 250° C.    -   oxidize adhesion layer        -   for example, RTA 2 minutes at 725° C. in O₂    -   deposit Pt electrode        -   for example, 2500A evaporated onto wafers at 310° C.    -   if used in process without an RTP FE crystallization step, then        an adhesion anneal may be desirable        -   for example, RTA 30 seconds at 725° in O₂.

The improved electrodes of the present invention can provide manybenefits. The electrodes can provide an improved performance, such asimproved switching characteristics for ferroelectrics (2Pr, 2Ec), andlarger NDRO signal for ferroelectrics. The electrodes can comprisesmoother electrode surfaces both before and after FE firing. This canprovide significantly improved yield, lower leakage current, andincreased breakdown.

The improved electrode forming process can also allow thicker electrodesto be used. This can reduce conductive loss in electrodes which candominate device performance at higher frequencies. The improvedelectrode forming process can provide larger process windows which arerelatively insensitive to RTP oxidation time, Ta thickness, Ptthickness, and Pt purity. The present invention can provide benefits fora variety of materials such as SBTN, SBT and BST.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the claims.

1. A method of forming a conductor on a substrate consisting essentiallyof the sequential steps of: depositing tantalum directly on a glasslayer of substantially uniform cross sectional thickness of thesubstrate; forming an adhesion layer of substantially uniform crosssectional thickness directly on the glass layer by oxidizing thedeposited tantalum to form oxidized tantalum using at least one of arapid thermal processor exposing the deposited tantalum to apredetermined temperature in an oxygen ambient environment or an in-situoxidation within a metal deposition system; and depositing a noble metaldirectly and only on the oxidized tantalum at a temperature at or above300° C. to form the conductor, and wherein the step of forming theadhesion layer comprises completely oxidizing the deposited tantalum. 2.A method as in claim 1 wherein the glass layer comprises silicon oxide.3. A method as in claim 2 wherein the step of depositing tantalum on theglass layer comprises evaporating the tantalum directly onto the siliconoxide.
 4. A method as in claim 1 wherein the step of depositing thenoble metal comprises evaporating the noble metal directly onto theoxidized tantalum.
 5. A method as in claim 1 wherein the step ofdepositing the noble metal comprises vapor deposition of the noble metaldirectly onto the oxidized tantalum.
 6. A method as in claim 5 whereinthe noble metal comprises platinum.
 7. A method as in claim 1 whereinthe step of depositing a noble metal on the oxidized tantalum comprisesheated deposition of platinum directly on the oxidized tantalum.
 8. Amethod as in claim 1 further comprising adhesion annealing the noblemetal with the oxidized tantalum.
 9. A method of forming a thin filmferroelectric device consisting essentially of the steps of: forming aconductor on a substrate as in claim 1, the conductor forming a bottomelectrode of the thin film dielectric device; forming a ferroelectriclayer on the bottom electrode; and forming a top electrode on theferroelectric layer.
 10. A method as in claim 1, wherein the glass layeris a wet oxidized portion of the substrate.
 11. A method as in claim 1,wherein the substrate is a silicon wafer.
 12. A method of forming aplatinum, electrode on a silicon oxide substrate of substantiallyuniform cross sectional thickness consisting essentially of thesequential steps of: forming an adhesion layer of substantially uniformcross sectional thickness comprising oxidized tantalum, wherein tantalumis first deposited directly on said silicon oxide substrate and thenoxidized using at least one of a rapid thermal processor exposing thetantalum to a predetermined temperature in an oxygen ambient environmentor an in-situ oxidation within a metal deposition system, the adhesionlayer being located directly on the silicon oxide; and directlydepositing the platinum only on the adhesion layer by heated depositionat a temperature at or above 300° C. to form the platinum electrode,wherein the electrode is a bottom electrode of a thin film dielectricdevice, and wherein the step of forming the adhesion layer comprisescompletely oxidizing the deposited tantalum.
 13. A method as in claim 12wherein the step of forming the adhesion layer comprises heated vapordeposition of tantalum directly on the silicon oxide.
 14. A method as inclaim 13 wherein the step of forming the adhesion layer comprisesoxidizing the tantalum in a rapid thermal processor before the platinumis deposited directly on the adhesion layer.
 15. A method as in claim 12wherein the step of directly depositing the platinum on the adhesionlayer comprises evaporated deposition of the platinum in an evaporator.16. A method of forming an electrode for a thin film capacitive deviceconsisting of the sequential steps of: deposition of a metal oxideadhesion layer of substantially uniform cross sectional thicknessdirectly and only on a top layer of substantially uniform crosssectional thickness of a glass substrate, wherein the metal oxideadhesion layer is formed by deposition of a metal followed by oxidationusing at least one of a rapid thermal processor exposing the depositedmetal to a predetermined temperature in an oxygen ambient environment oran in-situ oxidation within a metal deposition system; and deposition ofa noble metal conductor directly and only on the metal oxide adhesionlayer at a temperature at or above 300° C., wherein the electrode formsa bottom electrode of a thin film dielectric device, and wherein thestep of forming the adhesion layer comprises completely oxidizing thedeposited tantalum.
 17. A method as in claim 16 wherein the metal oxideadhesion layer comprises tantalum oxide.
 18. A method as in claim 16wherein the metal oxide adhesion layer is deposited by sputtering of ametal oxide target.
 19. A method as in claim 16 wherein the metal oxideadhesion layer is formed by MOCVD or MOD or sol-gel deposition of metaloxide precursors and subsequent thermal treatment.
 20. A method as inclaim 16 wherein the noble metal conductor is deposited by evaporation.21. A method as in claim 16 wherein the noble metal conductor isdeposited by evaporation onto a heated substrate.
 22. A method offorming a conductor on a substrate consisting of the sequential stepsof: directly depositing tantalum on a glass layer of substantiallyuniform cross sectional thickness of the substrate; forming an adhesionlayer of substantially uniform cross sectional thickness directly on theglass layer by oxidizing the deposited tantalum to form oxidizedtantalum using a rapid thermal processor exposing the deposited tantalumto a predetermined temperature in an oxygen ambient environment; anddepositing a noble metal directly and only on the oxidized tantalum at atemperature at or above 300° C. to form the conductor, and wherein thestep of forming the adhesion layer comprises completely oxidizing thedeposited tantalum.